It is often desirable to directly observe the surface of a high temperature melt. Such observation allows the user to optimize the various operating conditions associated with the melt. For example, by observing the melt surface the furnace temperature as well as the feed rates of the stock materials may be adjusted to optimize the melt rate. In furnaces using directed heat sources such as electron beam guns, the ability to observe the melt allows the user to control the operation of the electron guns, including the point of impact of the heat source. In addition, it is typically necessary to continuously view the melt surface during high rate vaporization of metal alloys in order to control the resultant vapor properties.
The observation of a high temperature melt is difficult for a variety of reasons. First, the temperature associated with the melt precludes the use of most common observation techniques since the high temperature rapidly damages electronics and optics alike. Furthermore, the temperature is typically high enough that connections to remote electronics become difficult due to the temperature effects on cables, interconnects, etc. Second, the dynamic range presented by a high temperature melt is typically greater than 10.sup.4, a range that exceeds the dynamic range of a charge coupled device (CCD) by about a decade. Third, at a high vapor rate condensing vapor can quickly obscure the viewing optics. Fourth, many of the materials of interest are corrosive. Fifth, there is typically very little space for mounting a viewing camera near the melt. Lastly, for many applications the viewing device must be mounted very near to the melt. Unfortunately, proximity to the melt further exacerbates all of the above constraints.
A number of different approaches have been taken to overcoming the problems associated with viewing a high temperature body exhibiting a wide range of temperatures and a correspondingly large light intensity range. Given that most recording equipment has a dynamic range of two to three orders of magnitude, typically it is necessary to compress the light intensity range to an acceptable level. A common approach is to electronically compress the intensity range. In this approach a detector detects the incoming light and outputs an electrical signal corresponding to the detected light intensity. The electrical signals output by the detector are then compressed and transformed into a format viewable and/or recordable by the user.
U.S. Pat. No. 4,726,660 discloses an optical approach to compressing a light intensity range. The disclosed compression technique utilizes a cholesteric liquid crystal notch filter that is configured to pass light at all wavelengths except for a relatively narrow wavelength band defining the filter's notch. The notch associated with this filter varies to a limited extent with the intensity of the incident light. The notch filter is used in combination with an interference filter to compress light intensity ranges.
A camera that can be used in a high temperature, corrosive environment to view a high temperature melt emitting radiation over a large dynamic range is therefore desired.